;;; If FUN has no physical environment, assign one, otherwise clean up
;;; the old physical environment, removing/flagging variables that
;;; have no sets or refs. If a var has no references, we remove it
;;; If FUN has no physical environment, assign one, otherwise clean up
;;; the old physical environment, removing/flagging variables that
;;; have no sets or refs. If a var has no references, we remove it